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Merged
merged 1 commit into from
Sep 5, 2025
Merged

mcp: avoid "null" when tools fail to return content #424

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10 changes: 7 additions & 3 deletions mcp/server.go
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Original file line number Diff line number Diff line change
Expand Up @@ -496,9 +496,13 @@ func (s *Server) callTool(ctx context.Context, req *CallToolRequest) (*CallToolR
Message: fmt.Sprintf("unknown tool %q", req.Params.Name),
}
}
// TODO: if handler returns nil content, it will serialize as null.
// Add a test and fix.
return st.handler(ctx, req)
res, err := st.handler(ctx, req)
if err == nil && res != nil && res.Content == nil {
res2 := *res
res2.Content = []Content{} // avoid "null"
res = &res2
}
return res, err
}

func (s *Server) listResources(_ context.Context, req *ListResourcesRequest) (*ListResourcesResult, error) {
Expand Down
12 changes: 6 additions & 6 deletions mcp/streamable_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -520,7 +520,7 @@ func TestStreamableServerTransport(t *testing.T) {
method: "POST",
messages: []jsonrpc.Message{req(2, "tools/call", &CallToolParams{Name: "tool"})},
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{resp(2, &CallToolResult{}, nil)},
wantMessages: []jsonrpc.Message{resp(2, &CallToolResult{Content: []Content{}}, nil)},
},
},
},
Expand All @@ -547,14 +547,14 @@ func TestStreamableServerTransport(t *testing.T) {
headers: http.Header{"Accept": {"text/plain", "*/*"}},
messages: []jsonrpc.Message{req(4, "tools/call", &CallToolParams{Name: "tool"})},
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{resp(4, &CallToolResult{}, nil)},
wantMessages: []jsonrpc.Message{resp(4, &CallToolResult{Content: []Content{}}, nil)},
},
{
method: "POST",
headers: http.Header{"Accept": {"text/*, application/*"}},
messages: []jsonrpc.Message{req(4, "tools/call", &CallToolParams{Name: "tool"})},
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{resp(4, &CallToolResult{}, nil)},
wantMessages: []jsonrpc.Message{resp(4, &CallToolResult{Content: []Content{}}, nil)},
},
},
},
Expand Down Expand Up @@ -592,7 +592,7 @@ func TestStreamableServerTransport(t *testing.T) {
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{
req(0, "notifications/progress", &ProgressNotificationParams{}),
resp(2, &CallToolResult{}, nil),
resp(2, &CallToolResult{Content: []Content{}}, nil),
},
},
},
Expand Down Expand Up @@ -624,7 +624,7 @@ func TestStreamableServerTransport(t *testing.T) {
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{
req(1, "roots/list", &ListRootsParams{}),
resp(2, &CallToolResult{}, nil),
resp(2, &CallToolResult{Content: []Content{}}, nil),
},
},
},
Expand Down Expand Up @@ -674,7 +674,7 @@ func TestStreamableServerTransport(t *testing.T) {
},
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{
resp(2, &CallToolResult{}, nil),
resp(2, &CallToolResult{Content: []Content{}}, nil),
},
},
{
Expand Down